A two-dimensional (or linear) ion-processing device such as an ion trap is formed by a set of electrodes coaxially arranged about a central (z) axis of the device and having predominant lengths in the direction of the central axis. Each electrode is positioned in the (x-y) plane orthogonal to the central axis at a radial distance from the central axis. The resulting electrode arrangement defines an axially elongated interior space of the device between opposing inside surfaces of the electrodes. In operation, ions may be introduced, trapped, stored, isolated, and subjected to various reactions in the interior space, and may be ejected from the interior space for detection. Such manipulations require precise control over the motions of the ions present in the interior space. The radial excursions of ions along the x-y plane may be controlled by applying a two-dimensional RF trapping field between opposing pairs of electrodes. The axial excursions of ions, or the motion of ions along the central axis, may be controlled by applying an axial DC trapping field between the axial ends of the electrodes. Additionally, auxiliary or supplemental RF fields may be applied between an opposing pair of electrodes to increase the amplitudes of oscillation of ions of selected mass-to-charge ratios along the axis of the electrode pair and thereby increase the kinetic energies of the ions for various purposes, including ion ejection and collision-induced dissociation (CID).
Ions present in the interior space of the electrode set are responsive to, and their motions influenced by, electric fields active within the interior space. These fields include fields applied intentionally by electrical means as in the case of the above-noted DC and RF fields, and fields inherently (mechanically) generated due to the physical/geometric features of the electrode set. The applied fields are not only governed by their applied operating parameters (amplitude, frequency, phase, and the like) but also by the fabrication and assembly, and resulting geometry and stability, of the physical components of the electrode structure. The inherently generated fields are often not intentional and often not desirable for optimal operation of the ion processing device. The inherently generated fields are also governed by the fabrication, assembly, geometry and stability of the electrodes. In particular, both applied fields and inherently generated fields are governed by the configuration (profile, geometry, features, and the like) of the inside surfaces of the electrodes exposed to the interior space. In advanced ion processing devices, the inside surfaces of the electrodes are typically hyperbolic with apices facing inwardly toward the central axis. Ideally, these inside surfaces are precisely machined with exceedingly close tolerances to accurately provide the intended profile (e.g., a hyperbolic sheet or other desired curved surface). Moreover, even when the inside surfaces are precisely machined, the positions of the electrodes relative to one another still need to be precisely oriented in the radial plane, and their orientations need to be maintained during operation, so that a given inside surface is not rotated, skewed, or otherwise out of orientation with the other inside surfaces. The positions of the electrodes also need to be accurately controlled and maintained relative to the z-axis so that the electrodes are precisely parallel to one another. For an electrode set of typical dimensions, the mechanical tolerance in the parallelism between an opposing pair of electrodes should be no greater than ±20 μm to obtain acceptable mass unit resolution.
Any differences in an electrical field relative to axial position along the central axis of the electrode set can adversely affect the desired response of the ions and consequently the performance of the electrode set as an ion-processing device. For instance, when the electrode set is employed as an ion-trap mass analyzer, non-uniformity in the field along the central axis can cause ions of the same mass-to-charge ratio to be ejected at different instances of time, resulting in a loss in mass resolution. Inferior fabrication, assembly, geometry and stability of the electrodes may lead to imperfect curvatures, improperly oriented electrode surfaces, and non-parallel electrodes. These problems may in turn cause non-uniformity in applied fields and unwanted inherently-generated fields. Moreover, the electrode set and its supporting components may be subjected to external forces from a variety of sources such as mounting stresses and strains, external thermal strains, handling or shipping loads, or creep of external components even after many years at the site of operation. Such external forces may distort the electrodes, alter the shapes of their surfaces, and cause the electrodes to lose their parallelism.
In view of the foregoing, there is a need for isolating electrode sets and associated components of ion processing devices from external forces. There is also a need to provide improved methods for fabricating and assembling such electrode sets and ion processing devices.